Conventional x-ray treatment of a tumor in a patient is carried out by planning the radiation angles and dosage by taking into consideration safety factors in respect to the patient's organs which would be in the path of the beam. The treatment plan assumes that the treatment equipment has certain capabilities. Accordingly, the current treatment practice assumes that the machine can cause a beam of selected rectangular shape and intensity to intersect a central fixed point in space from any solid angle. Therefore, the positioning of the patient and the use of multiple positions and multiple beam directions enable one to obtain integrated high doses on selected areas while maintaining low irradiation of other organs. Heretofore, control of the outline of the cross-section of the x-ray beam was accomplished by using jaw devices and control of the intensity of the beam was possible by using absorber plates or accelerator energy controls which provide uniform intensity across the beam cross-section. Irregular shape field boundaries are then obtained by mounting shadow blocks on a shadow tray and irregular intensity across the cross-section is obtained by use of wedge filters or compensating filters (which are shaped pieces of metal), all of which are inserted between the jaws and the patient. These devices naturally have to be changed at every angle.
My invention permits an entirely new method of treatment which eliminates the need for shadow blocks, wedge filters and compensating filters of the prior art and reduces the workload for the radiation technologists in treatment of the patient, while at the same time permits much improved precision in the two dimensional intensity distribution shaping of the resulting dose distribution in the patient. Furthermore, since my invention enables this beam shaping and intensity distribution control to be accomplished dynamically, it enables use of more effective treatment programs which would have been impractical in the prior art.
In conventional therapy, rectangular field shapes are formed by four motor driven jaws in the radiation head. Irregular field shapes for individual portals are then produced by mounting shadow blocks on a shadow tray between the jaws and the patient. The shadow blocks shield critical organs not invaded by the tumor. The radiation beam can be directed at the prescribed treatment volume from a single direction (single port therapy), from two or more directions (multi-port therapy), or the beam can be swept through an arc (arc or rotation therapy), all by rotating an isocentric gantry, for example. A cylindrical-shaped region of high dose is produced by a rectangular field in multi-port, arc or rotation therapy.
In multi-port therapy, the shadow blocks are changed for each beam angle. If the beam angle is not vertical, the shadow blocks must be locked to the shadow tray to avoid their falling off. Handling these blocks individually or on shadow trays is time-consuming. The shadow blocks are typically made by pouring a heavy metal into a pre-cut mold, which is also time-consuming. The shadow blocks can be heavy, difficult to handle, and dangerous if they fall on the patient or the radiotherapy personnel. In arc or rotation therapy, it is not practicable to change the shadow blocks continually or in small steps of beam angle. Also, this can require that the technologist go back into the shielded treatment room for each treatment field, a time-consuming process.
The usual treatment field shapes result in a three-dimensional treatment volume which includes segments of normal tissue, thereby limiting the dose that can be given to the tumor. The irradiation dose that can be delivered to a portion of an organ of normal tissue without serious damage can be increased if the size of that portion of the organ receiving such radiation dose can be reduced. Avoidance of serious damage to the organs surrounding and overlying the tumor determines the maximum dose that can be delivered to the tumor. Cure rates for many tumors are a steep function of the dose delivered to the tumor. Techniques are reportedly under development to make the treatment volume conform more closely to the shape of the tumor volume, thereby minimizing the product of volume and dose to normal tissue, with its attendant effects on the health of the patient. This other technique could possibly permit higher dose to tumors or can result in less damage to normal tissue. These techniques reportedly involve moving the x-ray jaws during treatment, scanning the x-ray beam or using multileaf collimators. Generally, in the prior art, multileaf equipment has not been capable of shaping internal regions of the field, e.g., islands and longitudinal peninsulas.
In a technique called dynamic therapy, one set of jaws is set to form a narrow (e.g., 4 cm) fan x-ray beam and the spread of the fan beam is varied by the second set of jaws to conform to the boundaries of the prescribed treatment volume as the beam is swept or stepped in angle around the patient and as the patient and associated table top are moved through the fan beam. A computer controls the movements of the table top in x, y and z, the gantry angle, the upper jaws during start and stop of the scan, the lower jaws throughout the scan, and the dose rate. The complexity is such that great care must be exercised in preparing for such treatments, which consumes considerable time.
A technique has also been proposed in which a narrow collimated lobe of x-rays is scanned over the treatment field, permitting production of irregular field shapes at selected beam angles. Because only a small fraction of the x-ray output is within the narrow lobe, the effective dose rate is low and the time to produce a portal field is hence long and multi-port treatment times are excessively long. Also, scanning individual fields is not readily applicable to arc and rotation therapy modes.
Machines have been built in which each of the lower pair of jaws is divided into a number (e.g., 5 to 32) of narrow bars called leaves. Each leaf may be about 8 cm thick (in the beam direction) to provide adequate attenuation of the x-ray beam (down to about 1%), about 0.5 to 1.5 cm wide and about 14 cm long physically (not SAD). Each leaf can be moved independently by a motor drive. This permits the production of irregularly shaped fields with stepped boundaries, thereby avoiding shadow blocks for many situations in portal therapy. The shape can be changed as the beam direction is swept in arc or rotation therapy. The disadvantage of this technique of replacing the lower jaws by a multiplicity of leaves is that each leaf is quite large and heavy, requiring a motor drive system which consumes considerable space. There is limited room in the radiation head for all these components so either sacrifices in performance are made (such as fewer leaves, limited field size) or the construction costs become large.
In a different technique, the conventional upper and lower pairs of jaws are retained and a set of leaves is mounted between the jaws and the patient. Each leaf moves in a plane, driven by a rotating cam or pushed by a form corresponding to the desired irregular field shape. In one early concept, each leaf was thick enough to attenuate the x-ray beam to the required level (to about 5% of unattenuated beam intensity), the ends and sides of the leaf forming a rectangular parallelpiped, hence the ends and sides were not aimed toward the x-ray source. In a recent concept, a multiplicity of small diameter rods forms a stack sufficiently thick to provide the required beam attenuation. Each rod can slide with respect to its neighbors. A form corresponding to the desired field shape boundary is used to push the assembly of rods so that their ends form a similar beam boundary. Since the rods are small in diameter, the radiation field boundary can be relatively smooth (very small steps) and tapered (focused) toward the source. However, varying the field shape as a function of beam angle without entering the treatment room can require a quite complex drive system because the large number of rods requires that they be driven enmasse instead of individually.
Wedge filters are pieces of metal which are tapered in one direction but of constant thickness in the orthogonal direction. They are used to produce a more uniform dose distribution in a treatment volume when it is irradiated from two directions which are less than 180.degree. apart. And they are used at any gantry angle as a crude compensation for the variation in depth from the patient's surface to the plane at treatment depth. In both cases, only an approximate correction of dose distribution in the treatment volume is achieved. Typically, standard wedges are used, with wedge angles of 15.degree., 30.degree., 45.degree. and 60.degree.. Intermediate angles are achieved by using two exposures per field, one with wedge filter, one without. Since manual insertion and retraction of wedges is laborious, fixed angle (typically 60.degree.) auto-retractable wedge filters have been developed. Essentially all wedged fields then require two exposures, one with the wedge filter, one without. This is a time-consuming process, especially in rotational therapy, since an extra gantry rotation is required.
Compensators, often termed compensation filters, are formed or assembled pieces of metal which are shaped to match the patient's demagnified anatomical shape so as to attenuate the x-ray beam by the amount that would have occurred if the patient thickness to depth of treatment plane were uniform. However, their use has been more limited because of the needs for custom shaping for each patient and manual insertion for each field.
Computed tomography (CT) images for treatment planning are typically obtained in successive planes which are normal to the patient axis. After transfer of these images, internal structures, target volume and patient surface can be outlined directly on the treatment planning computer display. However, in conventional radiotherapy, correction is required for divergence of the x-ray beam in the direction through the successive CT planes. This is a computation chore (beam's eye view) for the treatment planner and a mental visualization chore for the radiation therapist.